Ammonia (also known as “azane”) is a compound of nitrogen and hydrogen with the chemical formula NH3. Ammonia, as it used commercially, is often called anhydrous ammonia to emphasize the absence of water in the material. Because NH3 boils at −33.34° C. (−28.012° F.) at a pressure of 1 atmosphere, ammonia in liquid form must be stored under high pressure or at low temperatures. Although in wide use, ammonia is both caustic and hazardous.
One of the commercial uses for ammonia is as a process gas for semiconductor manufacturing. For example, ammonia is used for epitaxial deposition of semiconductor materials in a process called metal-organic chemical vapor deposition (MOCVD). MOCVD has become a major process in the manufacture of optoelectronics, such as light emitting diodes (LEDS).
Of the many gasses used in the manufacture of LEDS in MOCVD machines, ammonia is one of the most costly. This is due, in part, to the high purity required of ammonia process gas. For example, if moisture (H2O) or oxygen (O2) molecules are present in the ammonia gas, even in trace concentrations above a few parts per billion (ppb), then oxygen atoms can become incorporated into the crystalline structure of an LED device. Since ammonia is required at high flow-rates during the nitride-crystal growth process, even trace quantities of impurities in the gas can lead to a significant number of unwanted atoms being incorporated into the device. To combat this problem, state-of-the-art LEDS are made using ammonia that typically undergoes purification and contains no more than 1 ppb of moisture or oxygen.
Ammonia gas can be used in very high quantities by MOCVD machines. For example, a single MOCVD process chamber can consume about 10 tons of ultra-high purity ammonia gas per year, and a semiconductor production fab may have 50 MOVCD chambers or more. As such, the cost of ammonia and its disposal after it has been used are problematical.
FIG. 1 illustrates a prior art manufacturing system S including semiconductor manufacturing equipment M, a scrubber Sc and a general exhaust system E. The semiconductor manufacturing equipment M can be, for example, an MOCVD process chamber into which process gasses are injected in order to form epitaxial layers of semiconductor materials. The manufacturing equipment M has exhaust gasses, e.g. NH3, H2, N2 and other compounds at trace levels which are preferably flowed through the scrubber Sc to remove the NH3 before disposing of the remaining gasses in the general exhaust system E.
Verantis Environmental Solutions Group of Cleveland, Ohio sells a “Type MS Mini-Scrubber” which can be used to reduce ammonia content in the exhaust of semiconductor manufacturing equipment. The Mini-Scrubber uses a dilute sulfuric acid solution which is said to achieve a greater than 95% ammonia removal rate. However, it will be appreciated that scrubbers, such as the Verantis Mini-Scrubber, introduce their own complexities due to the use of consumables such as the sulfuric acid solution and the production of waste products that create a disposal problem.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.